Recent reports have suggested that Covid-19 has become markedly less lethal in the United States. Our analysis of death rates and infection fatality rates from Arizona, the U.S. as a whole, and New York City shows it isn’t, indicating that public health measures to reduce infections should not be relaxed.

Determining the true fatality rate can also help identify why more people with Covid-19 are not benefiting from advances in care.

As of late July, the state of Arizona was reporting a fatality rate of 2.1% among people who contracted Covid-19. That is markedly lower than reports in the spring from areas such as New York City, which saw fatality rates as high as 10%.

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The supposed reduction in lethality of Covid-19 has been largely attributed to improvements in treatments; earlier identification of Covid-19 infections, allowing for rapid medical intervention; and protection from infection of older higher-risk individuals. It also could be explained by increased testing, which would identify more mild and asymptomatic cases, leading to a lower reported fatality rate.

Knowing the true infection fatality rate (IFR), a key focus of research for one of us (D.L.R.), is essential in assessing whether regions are seeing improvements in the timeliness and effectiveness of Covid-19 treatment and, if they aren’t, rapidly identifying and implementing better practices.

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The infection fatality rate represents the percentage of all people infected with Covid-19 who die from the disease. Getting a solid handle on the IFR is critical for accurate Covid-19 hospitalization and fatality projections, which are needed to guide public health measures.

To gauge possible changes in the lethality of Covid-19, we made a conservative estimate of the infection fatality rate in Arizona, which has seen the majority of its cases and fatalities since late June. We chose Arizona based on its excellent Department of Health Services data reporting site and its state-of-the-art hospital care system. We compared our estimate to the best estimate of the infection fatality rate across the United States by the Centers for Disease Control and Prevention (CDC) from data obtained earlier in the pandemic, primarily the spring of 2020.

To determine the infection fatality rate in Arizona, we divided the percentage of the state’s population who had died from Covid-19 as of July 30, 2020, by the 12.9% of the population that was infected based on antibody testing between July 20 and July 26, 2020. Antibody testing captures the total percentage of the population that had been infected with Covid-19 from the beginning of the outbreak. We then calculated and applied a standard correction factor for the delay between case diagnosis and death. This yielded an infection fatality rate of 0.63%, which is not significantly different from the CDC’s best estimate of 0.65% for the U.S. in the Spring of 2020.

Patrick Skerrett / STAT Source: Douglas L. Rothman

A similar value, 0.68%, was reported from an extensive meta-analysis of published reports from the United States and other developed countries through May 2020.

To independently assess if Covid-19 has become less lethal, we compared the ratio of deaths to hospitalizations reported by Arizona with the ratio reported by New York City. This comparison is independent of accurately knowing the percentage of the infected population. We chose New York City because it was among the earliest and hardest hit regions in the U.S., so improvements in the effectiveness and timeliness of treatments since then should be easily detected. There was surprisingly little difference in this ratio overall and within age groups, as shown in the chart below.

Patrick Skerrett / STAT Source: Douglas L. Rothman

It is unlikely that Arizona is an exceptional case: other states experiencing large increases in cases and fatalities since late June have reported similar fatality rates.

If Covid-19 is not becoming less deadly, what explains the five-fold lower reported fatality rate in Arizona now compared to New York City in the spring? The most likely explanation is that the large increase in testing since the spring has increased the number of diagnosed cases several fold. The reported fatality rate is calculated by dividing the number of deaths by the number of diagnosed cases. A larger number of diagnosed cases due to more testing would decrease the reported fatality rate. This conclusion is consistent with a CDC report that the true number of infections was underestimated in the U.S. during the March-to-May period by as much as 10 fold.

Given progress in the diagnosis and treatment of Covid-19, why has there been no apparent improvement in the infection fatality rate? The two main possibilities are that the improvements that have been made in treating Covid-19 are not enough to make a detectable difference in the infection fatality rate, or that a large fraction of those who die of Covid-19 do not get to the hospital in time for successful treatment. We were, unfortunately, unable to find the fatality rate of patients who were hospitalized. Such data would help distinguish between these possibilities.

We recommend that all states implement procedures to track the ratio of deaths to hospitalizations, as well as implement random testing studies to accurately track infection fatality rates. Without this vital information, our ability to improve the health infrastructure to treat Covid-19 is handcuffed and may well lead to deaths that could otherwise have been prevented.

Douglas L. Rothman is a professor of radiology and biomedical engineering at Yale School of Medicine. Jessica E. Rothman is a graduate student in biostatistics at Yale School of Public Health. Gerard Bossard is a freelance writer.

Editor’s note: The article was updated to clarify the difference between the true infection fatality rate and the reported fatality rate.

  • I do not pretend to know anything, just asking questions- if the virus itself changed to become less lethal, wouldn’t the virologists see that by looking at it’s RNA?
    Another question -IS there a variant of the virus which IS less lethal? I read about one with a deletion where there are far, far fewer severe infections. Should we let that one loose here?
    Since the IFR has not changed since April or thereabouts- does this mean treatments are not really any better?
    Is there a country which truly has greatly improved it’s IFR? And most important question, HOW???? HOW????
    I realize the need to track and understand the epidemic, but I really want to know what treatments, if ANY, clearly work.

  • How do you explain the low mortality in several European countries then.
    For example, France has currently 5000 detected cases per day (how many real cases ? Maybe 10000), but only 20 fatalities. That’s close to 0.2 %
    In April there were 1000 fatalities per day but cases were maybe close to 50000, giving a death rate of 2% in April

  • Simplest way is to look at infection rate and death rate 14 days later. Why is there no exponential increase if Covid’s lethality isn’t decreasing.

  • Using Antibody testing to determine the total number of individuals that had COVID is flawed. There are numerous reports of people NOT having antibodies but having been infected. This is likely due to the body’s T cells eliminating the virus before the body needs to move on to creating antibodies to beat the virus. Therefore your total infected population is undercounted and the effective death rate below the .63 you project.

    • You are correct that antibody tests are an unreliable method of determining, because many recovered people do not have a detectable amount of antibodies in their bodies, but the reason you gave is slightly incorrect.

      Actually, the antibodies dissipate from the body a few months after the person has recovered from the disease. However, the T-cells remember the virus, and they will ramp up an immune response very quickly if the person becomes reinfected. Therefore, there is not one confirmed case where a person who recovered has gotten the disease a second time.

    • There are plenty of documented cases where a person was infected in the nasal/sinus tract only and did not show antibodies in the blood. In some cases, tests show antibodies in the nasal tract only, but these tests are difficult to do. Death rates in the overall population could go down if an increasing fraction of cases are confined to the nasal/sinus tract and do not result in severe disease or a positive blood antibody test – or in other words are more like a cold.

  • While by no means an attempt to diminish the severity of this virus on the percentage of our population that is vulnerable, one thing that no one is even considering are the number of deaths that are being listed as being coronavirus deaths that really aren’t. I live in NYS, and several nurses and other hospital staff have told me that deaths that being listed as due to Covid-19 that aren’t at all, as the hospitals get paid more for listing them as Covid-related. While I can’t verify whether this is true or not, they have no reason to lie, and I’m not the only one who has been told this. This should be investigated.

    • The only number which is the most accurate would be the number of deaths. Since so many were not permitted to be tested in the beginning and everyone stands behind the flawed testing to attempt to get a better picture of how many may have had it, it boggles my mind how anyone dare to guess at these number and they are guesses whether or not you base it on a control group. Until you can guarantee a number I suggest you put your efforts into treatment of the disease rather than trying to continue to keep people in fear. New York lost more than 30,000 lives and no other state has even come close to that number since.

    • While some cases may be incorrectly identified as covid, other countries do not have financial incentives to do so, yet they have higher covid mortality rates.

  • Do you have the IFR broken down by age groups? The IFR for the under-65 age group is significantly lower than above 65. Public health policy must take that into account or else you end up with policies like shutting down schools and businesses that are nonsensical.

  • 7.5% of the state’s population are age 65 years and over. Florida and Arizona are by far the most popular choices for retirees. I’m assuming nursing home staff are not social distancing, they get it from their kids, etc, bringing it into the facility and killing the people that live there. Utterly preventable.

    • Susan just HOW do you expect nursing home staff to live in a complete isolated bubble? Please tell us in YOUR wisdom how this is done. The average nursing home salaried staff is barely minimum wage. It is NOT high paid RN’s. Right now those nursing home staff you so cluelessly dismiss as careless and preventable are EXHAUSTED, OVERWORKED, UNDERPAID and do NOT have the time, energy nor RESOURCES (ie money) to go out partying anywhere. The nursing home staff you disparage have cared for the residents for years in many cases.
      Perhaps actually go WORK in a care home before ASS-U-ming

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